Big-Block Chevy Engine Build – Legacy Power

Some things are just a flash in the pan. A trend here today and gone tomorrow. The things that last and become embedded forever in our culture are those things that get it right from the start and just keep getting better. Such is the case with the almost 50-year-old big-block Chevy. Yeah, we know the small-block had a decade head start, but the fact that the big-block has been basically the same for almost five decades—that’s two generations of hot rodders—must mean that the General did something right, especially since that powerplant is still the main choice among the high-power bracket racers across the country today. That’s not the only thing that’s been on the job for decades. Veteran engine builder Mike Stine has almost as many years massaging those engines as they’ve been around; it’s the primary reason he chose one as his entry into the 2011 AMSOIL Engine Masters Challenge (EMC).

Mike has been building engines with his two brothers (and a lot of support from his wife!) since the early ’70s, and the Stine family has amassed a throng of circle track records and championships with their expertise in small-block Chevy and Ford powerplants. Mike is a Chevy man through and through, so when it came time to pick the engine for this build, he decided on the old porcupine engine: a big-block Chevy. The Mark IV engine that was the basis of this build has a past rooted as deeply in racing as the Stines themselves. In 1962, General Motors covertly began the design of a completely new revolutionary engine to replace the W block motors. Those old 348 and 409 Mark I engines were neat to look at and made decent torque, but just didn’t have the right stuff to make the top end power needed for racing. As the 1963 NASCAR season got fired up in Daytona, a slew of Chevys blasted around the track with an engine that was kept totally secret up to that point. It was the Mark II big-block Chevy “Mystery Motor,” and it shared almost nothing with its predecessor.

The new big-block was designed as a big-bore, short-stroke engine with beefy mains down below and high-flowing heads up top. As the Mark II was progressing on the racetracks in ’63 and ’64, there was a short-lived development program for a larger bore-center version dubbed the Mark III, but it was a no-go from the bean counters due to the high cost of retooling. By then, the fewer-than-50 Mark II engines produced were about used up as GM pulled the plug on their racing programs. Thankfully, the growing pains of the Mark II were worked out and an updated version was ready for placement in production vehicles for 1965. The Mark IV big-block’s reign began.

Since this particular engine was to be quite a bit different than the high-revving roundy-round motors they are used to producing, the brothers Stine, with Mike in the lead, aimed their rpm range a bit lower and wider than usual, as is required by the EMC rules. That meant this would be a serious torque monster. As Mike would tell you, there are two ways to build torque, otherwise seen as brake mean effective pressure (BMEP). The first option is to go for a small bore with a big stroke. First, let’s assume that the engine produces “x” amount of cylinder pressure. Let’s call it 200 pounds per square inch. With the 4.25-inch bore, that means there are 14.2 square inches of surface area on the piston crown for that pressure to push down on. A little math yields us 2,837 pounds of force shoving that piston down. That’s a ton of weight! Well, more than a ton, but moving on. That force is then transmitted through the arm of the crankshaft at an angle with the result becoming torque to twist that crank. The second design option is to use a big bore with a small stroke. Assume a 4.50-inch bore with that same 200 psi of pressure. That would result in 3,180 pounds of force. That cylinder pressure gives the same force as setting a Fox-body Mustang GT on top of your piston! You’d think that naturally this would produce more torque to the crankshaft, however, since the stroke is shorter, there is less of a lever arm to multiply that force, and it may in fact be less average torque per revolution than the first option. Choices, choices.

Mike went the route of a square combination, meaning the same bore and stroke. This played to his advantage as he was rules-restricted in lift and rpm, and history showed that a pretty long stroke was favored for average torque and a pretty good sized bore is needed to unshroud the valves enough to get air in there for high horsepower. Then there was the next decision. Rod length. Please, can we talk about rod length? I mean, you never see anything about it on the Internet, right? Ugh. OK, there is some science to it though.

The angular relationship between the rod and crank is ever changing and so as that force we were talking about before is pushing down on the piston, the rod is cocked over and making a compound angle with the crank arm. If you know the cylinder pressure at every point as the piston is going down, and you know the angles among the cylinder, rod, and crank arm, and you’re really good at calculus, you can actually determine the average force transmitted to the crank throughout a revolution. But—and there’s always a but—this is a four-stroke engine, so the crank has to make another revolution before the cycle is complete. Now you have to figure out what the importance of rod length is as it relates to the exhaust and intake flow, and the dwell at TDC and BDC, and so on. The long and short of it is that, in general, a long rod tends to benefit a bad cylinder head and a short rod tends to favor a good cylinder head. And Mike’s head is a good one, so he opted for a stock-length BBC rod.

Speaking of cylinder heads, it was the top side of the Mark II “Mystery Motor” that really made the looky-loos of the ’60s scratch their noggins when Junior Johnson and Johnny Rutherford popped their hoods for the first time. Gone were the trademark “W” shaped valve covers in favor of some massive aluminum boxes to cover up the valvetrain. Once the valve covers were popped off, passersby gawked at the rocker studs that were pointed every which way. By canting the valve angles so that they opened away from the cylinder walls and toward the center of the combustion chamber, Chevy was able to stuff huge valves in there for big flow numbers and minimal shrouding. Of course, all those watchers were able to spy was a bunch of studs sticking out in different directions like a bunch of porcupine quills. Hence its nickname, “porcupine.”

But back to our story. Mike chose a set of small-runner Brodix castings for this jewel. To get that air mass into the engine requires a certain amount of velocity, and it is a combination of cross-sectional area and airflow that determines the velocity. The runners were about 290 cc, and figuring an average port length of 6 inches would yield a mean cross-sectional area (MCSA) of 2.95 square inches. These little babies were flowing a killer 384 cfm at .700-inch lift, so they were hauling the mail with an average port velocity of about 315 feet per second. That is considered a really good target air speed for the rpm range in question (2,500 to 6,500 rpm). To make all that airflow mojo happen, Mike enlisted the help of the legendary Joe Patelle at HVH Cylinder Heads in Knoxville, Tennessee. HVH has been porting heads since the old porcupine had soft quills, and Mike says: “HVH has been doing all of our heads for probably the last four or five years.” Once he received the heads, he did the valve job in-house to finish them off. Mike mentioned that he’s played with different valve job angles in the past but for a relatively low-lift and low-rpm application like this he felt a standard 45-degree valve job would be the best bet.

The other part of the top end equation is the intake manifold. Since the heads had oval-shaped raised runners, there wasn’t a whole lot out there Mike had to choose from, so like any good machinist, he made something else fit. Starting with an Edelbrock rectangle-port single-plane intake, he added about a quarter inch of epoxy to the floors, then matched the shape of the cylinder head by grinding out the intake to an oval shape not much bigger than a golf ball. That didn’t mean they were a restriction though. “The manifold needs to flow about 5 percent more than the cylinder head,” Mike says. With the manifold flowing right, they like to stick the head back on the flow bench to see if there are any changes. “We actually like to flow the cylinder head with the manifold on it, and the numbers should come out the same with or without the manifold.” Typically intake manifolds like this end up with some sort of carb spacer since they are usually running a carb. So what happens when running something like FAST XFI fuel injection? Well, Mike tried a bunch of different carb spacers again to see what worked. He’s got about six versions ranging up to 3 inches thick, but settled on a simple 1-inch-thick open spacer to give a little more plenum volume and to straighten out the air as it passes through the FAST throttle body.

Speaking of EFI, this was Mike’s first foray into fuel injection and it was not without drama. He started out wiring up the system, as the XFI comes with a fully terminated harness that is labeled to keep it idiot-proof. Once he got it up and running, they had trouble on the dyno; going to wide open throttle, the TPS would show 100 percent then suddenly drop back to 75 percent on the computer. They chased wires and connections but finally went to the local auto parts store and got a replacement for every single sensor. Once back on the pump, it turned out it was simply a bad throttle position sensor that was giving out a bad signal. Just one of those things. That being said, Mike did praise the XFI: “The fuel injection is really neat to work with. It’ll do exactly what it says it does. If you tell it you want 13:1 air/fuel ratio, it’ll give you 13:1 air/fuel ratio.”

To run in the 2,500 to 6,500 rpm range that the engine was intended for and to do so with EFI and not a carburetor, Mike had to completely change the way he picked the cam for this Rat. “Our circle track engines use anything between a 106 and 108 lobe separation, and we run the engines in a certain rpm range, which is a lot higher than what this engine runs.” For a broad torque range and with fuel injection, Mike likes to really shrink up the duration and open the lobe separation to 110-112 degrees. Mike says he’s been running Jesel beltdrives and rocker arms for a number of years as the quality is top notch. The beltdrive has the advantage of being able to advance or retard the cam easily by just quickly removing the water pump to gain access to the adjustment bolts. Camshaft endplay on the solid roller is also set easily by adding or subtracting shims between the timing cover and an adapter mounted on the nose of the cam. Typical endplay is targeted at .002 to .004 inch. Tight endplay not only keeps the lifters lined up with the lobes, but since the timing gear on the distributor is helical where it meshes with the cam, it keeps the dizzy phased correctly. With a cam design finally in mind, Mike’s contact at COMP ground him a shiny new solid-roller cam to make the lumpy-lump noise at the right time.

With the cam beboppin’ along at the right pace and the MSD box burnin’ up a bunch of VP100 gas, the exhaust note through the Beyea headers and Flowmaster mufflers was just perfect. On the DTS dyno at the University of Northwestern Ohio, Mike’s own “Mystery Motor” lifted the power needle to the tune of 726 angry horses with 621 pounding feet of torque. No doubt that this Mark IV was one prickly pump gas performer that packed a preponderance of potent power.

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